Last night a cold front rolled through Chicago, and lucky for us art consultant Amy King was on the lakefront and stopped to shoot an amazing 5-second timelapse as a low-hanging roll cloud moved ominously down the shoreline. So, what’s a roll cloud? Meteorologist Cheryl Scott explains:

What is a Roll Cloud and how does it form? It’s a low, horizontal, tube-shaped cloud. It is formed by winds changing speed/direction when the air temperature reverses its state (resulting in warm air on top of cool air). The shear in the atmosphere sets up a rolling motion, think [of a] rolling pin used in a baking.

You can read a bit more about roll clouds—also called an Arcus Cloud—on Wikipedia. (via @kingartcollective)